Enclosed lens retroreflective sheeting generally comprises reflective sheeting having a polymer matrix thereon, with glass beads embedded in the matrix. A mirror or reflective surface, generally formed from a metallic vapor coat or the like, is formed on a back side of the polymer/bead composite. In typical operation, light passes through the beads, which individually act as lenses focusing the light and directing same against the mirror surface. The light is then reflected back through the beads, and toward the source. Typically, the mirror surface is separated from the glass beads by a spacing layer coat or spacecoat, which provides for a preferred focal length between the beads and the reflective surface. It is noted that one reason such embedded lens arrangements are useful, is that incident light rays are focused onto the reflective layer irrespective of whether the front of the sheeting is wet or dry.
The elements of a typical enclosed or embedded lens retroreflective sheeting are: lens arrangement (beads imbedded in polymer), spacing layer (spacecoat), and reflector surface (vapor coat). The sheeting may include other elements such as an outer protective layer, and/or an adhesive layer for mounting. Herein the term "spacecoat" is meant to generally refer to the resin which provides for a separation between the embedded lenses and the reflective coat, regardless of the process of formation. The end product will generally be referred to as an enclosed (or embedded) lens retroreflective sheeting, again regardless of the process of its formation.
In a typical application, the reflective surface is formed as a layer having a plurality of cupped or concentrically coated portions or concave portions, one each of which is in association with each bead or embedded lens. The concentrically coated portions facilitate a desired reflection of light which has passed through the lenses, regardless of the direction from which the light initially impinges onto the sheeting. In part, the cupped construction of the mirrored surface ensures that much of the light reflected by the retroreflective surface is directed back toward the source.
Enclosed lens retroreflective sheeting and the use of glass beads to provide for reflex light reflectors are described in Palmquist et al., U.S. Pat. Nos. 2,407,680; May, 4,626,127; Tung et al., 4,367,920;, Tung et al., 4,511,210; and, Tung et al., 4,569,857; these references being incorporated herein by reference.
From the above, it will be apparent that the nature of the spacecoat is very important. In particular, the spacecoat must be of a material than can be precisely applied, and which will be dimensionally stable in use. By "precisely applied" it is meant that application with precise control of thickness and conformation to the beads is obtainable. By "dimensionally stable" it is mean that the spacecoat should be sufficiently strong and durable (i.e. stable) over time, to maintain proper spacing and relative orientation between the individual glass beads, and the cup-shaped reflective surface. Any substantial deformation of the spacecoat, will lead to significant reduction in reflective ability (or power) of the retroreflective material.
In a typical application, enclosed lens retroreflective sheeting is applied to a substrate, such as wood, plastic, or metal, typically used to form a highway sign, license plate, or safety sign. Retroreflective material, when so applied, makes the objects formed from the substrate more conspicuous at night.
In some instances, it is desired to emboss a substrate having a retroreflective surface thereon. For example, a sheet of metal license plate material having a reflective surface thereon may be embossed to provide for conspicuity. Typically, the embossed letters or numbers are painted or otherwise colored to provide for greater contrast with reflective background. In some instances the colored symbols may be covered by an outer layer of a transparent thermosetting polymeric resin.
If the spacecoat is not substantially dimensionally stable, significant loss of reflective ability will occur as a result of the embossing. Also, exterior durability will diminish. That is, the spacecoat will tend to crack, wrinkle, split, fracture or peel, along points of a stress associated with the embossing. It is noted that the same would be likely for any substantial bending of the substrate, not merely for embossing.
Past polymeric compositions used for spacecoats have been less than completely acceptable with respect to this dimensional stability. That is, substantial distortion of the reflective surface readily occurs, especially if embossing or the like is conducted on the coated substrate: This has led to substantial loss of reflective power for the retroreflective surface. Further, cracks or splits associated with the embossing have formed sites at which deterioration of the retroreflective surface can begin to occur, leading to a shorter product lifetime than desirable. What has been needed has been a polymer composition which provides for relatively high dimensional stability of the spacecoat.
It will be readily understood that a polymer composition which provides for the above related characteristics when used as a cured spacecoat would probably have utility in other, non-embedded lens, applications wherein dimensional stability is important.